Originally, when I saw these two cards in the same deck it seemed like poetry in motion.
Spellbound Dragon discards some high-value card, like Emrakul, and I put it on top of my library with My Library. I draw it again next turn and repeat, smacking someone for 18 every time the Dragon swoops in.
This "combo" doesn't actually work.
1) The card is still being discarded, but the discard is modified by Library of Leng.
2) The information on the card being discarded is never revealed to other players
3) Because the information is never revealed, it cannot be used to affect the game state
At least, this was how it was explained to me. I guess it makes sense…theoretically I could just be discarding a Mountain and saying its was Emrakul. And at no point do any of the cards instruct me to reveal the contents of the card.
Resolved…I actually have to put the card in my graveyard to get the effect I want from Spellbound Dragon.
And no, I don't have any Comprehensive Rulebook quotations to back this up. I don't even know where I would start looking inside that mammoth tome. If anyone knows where it says this, please sent the quote and I'll have it branded on the back of my skull*.
To all the Johnnies out there, keep tinkering!
* with washable marker over a napkin.